The present invention relates to improvements in boilers of the type known, for example, from Swiss Pat. No. 485,182 and German Pat. No. 1,778,880. Although these boilers meet the requirements placed upon them as regards the possible heat utilization, the practical realization and an economical manufacture present considerable problems for which reason these boilers in all probability will not be commercially accepted. In this connection, the embodiments of the systems according to the patents in which U-shaped or approximately U-shaped sheet metal profiles are mounted on a cylindrical surface on the inner wall thereof and the profiles are welded together along the leg edges thereof, are of a special interest. Conversely, such boilers are unsuitable for use without difficulties over wide temperature ranges, especially low temperature ranges of, for example, between 30.degree. and 60.degree. C, on account of the risk of corrosion involved therewith.
In case of an excessive subdivision of the all-over discharge cross section, a high welding expenditure arises, involving the difficulty of no longer having adequate space between the sheet metal profiles for the welding tools; conversely, the basic regions of the U-profiles directed against the actual combustion chamber become so constricted, partly even pointed, that extremely unfavorable heat transfer conditions would arise if a protective combustion chamber sleeve were to be used on account of the scaling risk of the profiles. If, conversely, correspondingly large interspaces are provided to permit access to mechanical welding tools which nowadays offer the only alternative for an economical manufacture, the all-over discharge cross section is not sufficiently divided and the absorbed heat amounts can no longer be adequately discharged through the material webs. This would result in high exhaust gas temperatures and a scaling risk, the latter especially in the absence of a special combustion chamber sleeve, as is the case with the above-discussed and prior known embodiments.
Although the conception of such a formation of the fuel gas flues with the boilers of the afore-mentioned type in principle is a good one, these boilers do not satisfy the need for an economical and practical possibility of manufacture, for a long operating life and for optimum heat transfer conditions and a favorable corrosion behavior.